This invention relates to a method of producing ceramic fiber with a mullite fraction (3 Al.sub.2 O.sub.3.2SiO.sub.2).
The aluminosilicate fiber which is a known ceramic fiber and is used in various fields of application as a refractory material is generally produced by the melting-fiberizing method. In such a method, the blended raw material, consisting of silica sand and natural kaolin clay, sintered materials, or alumina, is melted in an electric furnace, and after the melted mixture is adjusted to a viscosity suitable for fiberizing, the said melted mixture is extracted in fine streams. The fiberizing is completed by blowing compressed air or by utilizing the centrifugal force of a rotating device, so that the material is in a vitreous (non-crystalline) state as a result of sudden cooling which occurs during the fiberizing process.
The range of the weight ratio of alumina (Al.sub.2 O.sub.3) to silica (SiO.sub.2) is most typically from 45:55 to 52:48. Although there have been attempts to produce a material with a higher alumina content for use as a high-temperature refractory material, along with the increase in the alumina content, the melting temperature becomes higher, and the fiberizing also becomes more difficult. As a result, the so-called shots (grainy forms resulting from failure in fiberizing) tend to increase. Therefore, the best ratio attained so far in this respect is 65:35.
Although aluminosilicate fiber is widely used as a high-temperature refractory material, this substance produces crystals at a higher temperature as a stabilized phase of the compositional components.
The crystallization of mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) starts to take place at about 950.degree. to 980.degree. C., so that at 1,200.degree. C., the theoretical amount is almost attained within several tens of hours. The inventor has discovered that, as the crystallization of the mullite progresses, the SiO.sub.2 ratio in the remaining glass phase of the material increases, and that cristobalite (SiO.sub.2) starts to appear in the form of crystals.
There will be several disadvantages if a portion of the SiO.sub.2, which existed as aluminosilicate (aluminum silicate) glass, is transformed into free silicic acid along with the generation of the above-mentioned crystals. The first disadvantage is the adverse effect of this material on the human body. Although glass fiber which contains glass-type aluminosilicate fiber and silica is not as yet recognized as being particularly hazardous to the human body, inhalation of dust of free silicic acid substances such as silica sand, etc. may cause silicosis.
At present, it has not been determined that fiber involving the crystallization of cristobalite is hazardous; however, attention must be paid to this material in view of prevention of possible danger to human health.
The second disadvantage is the resistance of this material to chemical corrosion. Dust containing alkaline metal oxides (Na.sub.2 O, K.sub.2 O), etc., flows along with the gas inside a furnace, and the dust eventually adheres to the stuctural material of the furnace, thereby causing corrosion. Such dust is generated from the ash portion of the consumed fuel in the case of many industrial furnaces, and in the case of the iron/steel production industry or the iron casting industry, the dust is usually generated from the components of the thermal insulation materials used to maintain the surface temperature of the melted materials. Free silicic acid is vulnerable to such corrosion. Since the specific surface area of a fibrous thermal insulating material is extremely large, the vulnerability to the effects of such corrosion also becomes extremely high.
The third disadvantage is the increased brittleness of the fiber. This increased brittleness is not highly conspicuous at the mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) crystallization stage, which is the first stage of the above-mentioned crystallization; however, the brittleness increases drastically as a result of the generation of cristobalite (SiO.sub.2) crystals, and thereby becomes highly vulnerable to pulverization. This phenomenon is understood to be due to the fact that cristobalite undergoes a drastic shift from alpha-cristobalite (low temperature type) to beta-cristobalite (high temperature type), or vice versa, in conjunction with a great change in volume in the vicinity of 1250.degree. C.
Since such generation of free silicic acid is undesirable, it is possible to attempt to produce a fiber having a composition such that no free silicic acid will be generated, i.e. production of a fiber with a composition in which the SiO.sub.2 fraction is less than the mullite fraction (3Al.sub.2 O.sub.3.2SiO.sub.2), which is 72 weight percent Al.sub.2 O.sub.3 and 28 weight percent SiO.sub.2. However, such production is highly difficult, as mentioned earlier, as long as the melting fiberizing method is to be employed.
Various production methods have recently been developed in order to cope with this problem. These methods generally are classified as the precursor fiberization method. According to these methods, fiberization is conducted after adding plasticizers, etc., as required, to a solution of organic and inorganic salts of aluminum and silicon, the salts respectively transforming into Al.sub.2 O.sub.3 and SiO.sub.2, as a result of thermal decomposition. The thus obtained fiber is further thermally decomposed and fiber having an Al.sub.2 O.sub.3 -SiO.sub.2) type composition is finally produced.
According to this method, it is possible to obtain fiber of nearly all types of composition. However, the fiber produced in this case is the so-called polycrystalline fiber in which the fine crystals ar bonded to one another through pores during the production process. Hence, such fiber generally has the disadvantage of having a low degree of resistance. Further, such a method involves complex processes and requires accurate control, thereby inevitably involving the disadvantage of extremely high production costs.